On Aug 31, 2009, at 11:13 AM, Les Mikesell <lesmikesell@xxxxxxxxx>
wrote:
> Chan Chung Hang Christopher wrote:
>>
>> Oh sorry, I have never argued about eight drive systems years ago
>> (didn't have them then, too poor) and there is no argument about
>> raid1+0
>> being the way to do it beyond four drives. It is too obvious that
>> stripping three drives and then mirroring them is more risky than
>> making
>> three mirrors and then stripping them. Any argument then about
>> whether
>> one should do raid0+1 were really limited to those who had four drive
>> systems and never thought beyond four drives.
>>
>> So it is really moot unless one ignores the obvious or fails to
>> think.
>>
>>> "Another difference between the two RAID configurations is
>>> performance
>>> when the system is in a degraded state, i.e. after it has lost one
>>> or
>>> more drives but has not lost the right combination of drives to
>>> completely fail."
>>>
>>> RAID 1+0 is still more secure."
>>>
>> Hear, hear. Man, I should leave the 90s back there.
>
> But note that drive capacity has gone up too, often eliminating the
> need
> for many-disk arrays. For example, you can go up to 2TB on a single
> drive, so a simple RAID1 mirror may be all you need, and if you can
> arrange the mount points to match the use pattern you may get better
> performance out of several separate raid1 partitions where the heads
> can
> seek independently instead of essentially tying them all together in a
> single array. A many-disk array may do better on artificial benchmarks
> accessing one big file, but that's not what most computers actually
> do -
> and raid1 has the advantages of not slowing down when a member fails
> and
> you can recover the data from any single drive.
Ah the larger capacity is double sided as it also increases the chance
of a double failure in a single parity RAID as the resilver time is
also substantially increased, so if doing parity on large drives you
should really go for RAID6, which unfortunately means you really need
5 drives over 3 to get the same economy as a RAID5.
-Ross
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